This invention relates generally to television display systems and particularly to a novel system for displaying a television picture having a 4.times.3 aspect ratio on a display screen having a 16.times.9 aspect ratio.
Most television cathode ray tubes (CRTs) are configured to display video signals formatted with an aspect ratio (AR) of 4.times.3. The AR is determined by the ratio of the horizontal dimension or length of the display to its vertical dimension, or height. For many years motion pictures have had exaggerated horizontal dimensions, and resultant displays that departed from 4.times.3. There are also some television systems that have CRTs suitable for displaying video signals with different ARs from the conventional 4.times.3 AR. A commonly discussed television video display AR is 16.times.9 with other ARs also being considered. Displaying a 16.times.9 AR video signal on a conventional 4.times.3 AR television CRT is readily accomplished by simply not scanning some of the upper or lower portions (or both) of the CRT. Similarly, a TV receiver having a CRT of 16.times.9 AR, may display a 4.times.3 AR video signal by not scanning portions of the left or right (or both) sides of the CRT. Some manufacturers include 4.times.3 scanning circuitry and add side panel curtains to block off the unscanned portions of the 16.times.9 AR CRT when receiving a 4.times.3 AR video signal.
In the system of the invention the full viewing surface of a 16.times.9 AR CRT is scanned when displaying a 4.times.3 AR video signal. Distortion of the displayed image is avoided, by compensating techniques since the image will be developed by horizontally scanning the CRT over a proportionately larger distance. These compensating techniques generally compress the signal to form a converted video signal so that it may be scanned on a portion of a larger AR display without substantial geometric distortion. In one version of the invention a decimation technique is used to compress the image signal and in another a clock speed up technique is used.